On the other hand, \(M\)  can be expressed as an integral over the
enclosed volume of certain components of stress energy tensor
\(\mathcal {T}_{ab}\)  [1]},
\(M=2 \int (\mathcal {T}_{ab} -\frac{1}{2} \mathcal {T} g_{ab})n^a\xi ^b d\Sigma .\) 
